ZrO2 Solid Electrolyte Aided Investigation on Electrodeposition in Na3AlF6-SiO2 Melt
GAO Yunming1,2(), HE Lin1,2, QIN Qingwei1,2, LI Guangqiang1,2
1.The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 2.Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China
Cite this article:
GAO Yunming, HE Lin, QIN Qingwei, LI Guangqiang. ZrO2 Solid Electrolyte Aided Investigation on Electrodeposition in Na3AlF6-SiO2 Melt. Acta Metall Sin, 2022, 58(10): 1292-1304.
Electrodeposition of silicon from a cryolite-based melt is a possible solution for the mass production of silicon with high purity. Currently, deposition of Si from dissolved SiO2 in cryolite-based melts occurs primarily in a graphite crucible using graphite and quasi-reference electrodes, resulting in series of problems, such as CO x emissions due to carbon participation, non-significant peak positions on cyclic voltammetry (CV) curves due to melt electronic conduction, various reference standards of metal deposition potential, and insufficient investigations on electrode reaction mechanism due to melt composition complexity. In this work, a novel three-electrode electrochemical cell with Pt, O2(air)|YSZ reference (RE), and counter electrodes (CE) was constructed using a Y2O3 stabilized ZrO2 solid electrolyte (YSZ) tube, CV and potentiostat electrolysis tests were performed on Ir wire working electrode in Na3AlF6-5%SiO2 (mass fraction) melt under the conditions of a complete carbon-free and 1323 K. The precipitation potentials of related metals on the cathode in the melt were investigated, and the electrodeposition law in the melt at different potentials was analyzed, using a combination of thermodynamic theoretical calculations, SEM observation, and EDS analysis. The results show that Si can be deposited on the Ir wire in a single step, and its peak potential is about -1.65 V on the CV curve, while the deposition potentials of Al, Na (Zr) are all negative than -1.8 V and increase negatively in turn. During potentiostatic electrolysis, intermetallic compound particles of Zr5Si4 are observed to generate at -1.8 V or -2.0 V, with a generation potential of -1.7 V to -1.8 V. The deposited Si, Al, and Na metals are mainly derived from oxygen-containing compounds produced by the Na3AlF6-SiO2 melt itself but Zr metal from the ZrO2 of the corrosion of YSZ tubes by the melt. The precipitation potentials of related metals (or intermetallic compounds) relative to Pt, O2(air)∣YSZ RE agree well with thermodynamic calculations.
Fig.1 Schematic diagram of electrochemical cell device
Fig.2 Curve of open circuit potential vs time for Na3AlF6-5%SiO2 melt at 1323 K (RE: Pt, O2(air)| YSZ)
No.
Chemical reaction
ΔGΘ / (kJ·mol-1)
EΘ / V
E / V
1
2Fe + 1.5O2 = Fe2O3
-483
0.84
-0.79
2
3Fe + 2O2 = Fe3O4
-696
0.90
-0.86
3
2Fe + O2 = 2FeO
-358
0.93
-0.88
4
4Na(g) + O2 = 2Na2O
-429
1.11
-1.07
5
Si + O2 = SiO2
-675
1.75
-1.70
6
2Al + 1.5O2 = Al2O3
-1255
2.17
-2.12
7
Zr + O2 = ZrO2
-842
2.18
-2.13
8
2Y + 1.5O2 = Y2O3
-1524
2.63
-2.59
9
Zr5Si4 + 5O2 = 4Si + 5ZrO2
-3550
1.84
-1.79
10
Zr5Si4 + 9O2 = 4SiO2 + 5ZrO2
-6248
1.80
-1.75
Table 1 Chemical reactions involving oxides and standard Gibbs energy (ΔGΘ) and potential data at 1323 K[33]
Fig.3 Cyclic voltammetry (CV) curves of Na3AlF6-SiO2 melt on the Ir WE at the scan rate of 30 mV/s (RE: Pt, O2(air)|YSZ)
Fig.4 Curves of current vs time during potentiostatic electrolysis for Na3AlF6-SiO2 melt (RE: Pt, O2(air)|YSZ)
Fig.5 SEM images of cross section (a1) of the Ir electrode and local magnification (a2), sediment particles (b1) at the bottom of YSZ tube and local magnification (b2) after electrolysis for pure Na3AlF6 melt at -2.2 V
Point
Mass fraction / %
Possible phase
(atomic fraction / %)
O
Ir
Zr
Au
1
25.97
74.03
ZrO2
(66.67)
(33.33)
2
3.56
80.25
16.19
Zr + ZrO2
(18.79)
(74.27)
(6.94)
3
25.97
74.03
ZrO2
(66.67)
(33.33)
4
80.86
3.80
15.35
Ir-Zr
(77.87)
(7.71)
(14.42)
5
79.34
8.18
12.48
Ir-Zr
(72.96)
(15.84)
(11.20)
6
83.63
16.37
Ir
(83.97)
(16.03)
7
25.97
74.03
ZrO2
(66.67)
(33.33)
Table 2 EDS results corresponding to the points in Fig.5
Fig.6 SEM images and scanline results of the Ir electrode section after electrolysis for Na3AlF6-5%SiO2 melt at constant potentials of -1.4 V (a), -1.6 V (b), -1.8 V (c), and -2.0 V (d)
Point
Mass fraction of element / %
Possible phase
(atomic fraction / %)
O
Ir
Zr
Si
Al
Fe
Au
1
7.61
19.43
32.85
1.76
38.36
Fe-Ir, ZrO2
(40.88)
(8.69)
(30.97)
(2.71)
(16.75)
2
55.56
44.44
Ir
(56.16)
(43.84)
3
72.67
1.31
13.22
12.79
Fe-Si-Ir
(52.04)
(6.44)
(32.58)
(8.94)
4
48.14
1.82
50.04
Ir-Si
(43.99)
(11.39)
(44.62)
5
48.74
2.37
48.89
Ir-Si
(43.27)
(14.37)
(42.35)
6
42.81
6.02
51.16
Ir-Si
(31.96)
(30.77)
(37.27)
7
44.33
11.81
43.86
Zr5Si4
(43.04)
(37.24)
(19.72)
8
40.30
2.59
6.90
1.17
49.04
Si x Al y Zr z Ir m
(27.01)
(3.66)
(31.64)
(5.61)
(32.08)
9
44.83
5.45
0.32
49.40
Si x Al y Ir z
(33.80)
(28.13)
(1.73)
(36.34)
10
39.00
6.98
6.14
0.22
47.66
Si x Al y Zr z Ir m
(27.12)
(10.22)
(29.24)
(1.07)
(32.35)
11
44.51
5.62
0.38
49.50
Si x Al y Ir z
(33.22)
(28.70)
(2.02)
(36.05)
Table 3 EDS results corresponding to the points in Fig.6
Fig.7 SEM images of cross section of the Ir electrode vicinity after electrolysis of the melt at constant potentials of -1.4 V (a), -1.6 V (b), -1.8 V (c), and -2.0 V (d)
Point
Mass fraction of element / %
Possible phase
(atomic fraction / %)
Zr
Si
Au
O
1
74.03
25.97
ZrO2
(33.33)
(66.67)
2
74.03
25.97
ZrO2
(33.33)
(66.67)
3
41.77
11.15
41.39
Zr5Si4(s)
(29.77)
(25.82)
(13.66)
4
40.16
11.07
42.32
Zr5Si4(s)
(27.76)
(24.84)
(13.55)
5
42.11
11.46
40.46
Zr5Si4(s)
(29.36)
(25.95)
(13.07)
6
41.86
10.85
40.07
Zr5Si4(s)
(27.82)
(23.41)
(12.33)
Table 4 EDS results corresponding to the points in Fig.7
Fig.8 Standard Gibbs free energy ΔGΘ and temperature T of oxidation reactions for Zr and Zr5Si4
1
Xu J L, Haarberg G M. Electrodeposition of solar cell grade silicon in high temperature molten salts [J]. High Temp. Mater. Proc., 2013, 32: 97
doi: 10.1515/htmp-2012-0045
2
Elwell D, Feigelson R S. Electrodeposition of solar silicon [J]. Sol. Energy Mater., 1982, 6: 123
doi: 10.1016/0165-1633(82)90014-4
3
Elwell D, Rao G M. Electrolytic production of silicon [J]. J. Appl. Electrochem., 1988, 18: 15
doi: 10.1007/BF01016199
4
Sokhanvaran S, Barati M. Electrochemical behavior of silicon species in cryolite melt [J]. J. Electrochem. Soc., 2014, 161: E6
doi: 10.1149/2.042401jes
5
Sokhanvaran S, Danaei A, Barati M. Determination of cell potential for silicon electrodeposition [J]. Metall. Mater. Trans., 2014, 1E: 187
6
Sokhanvaran S, Thomas S, Barati M. Charge transport properties of cryolite-silica melts [J]. Electrochim. Acta, 2012, 66: 239
doi: 10.1016/j.electacta.2012.01.077
7
Jia M, Lai Y Q, Tian Z L, et al. Electrodeposition behavior of silicon from Na3AlF6-LiF melts [J]. Acta Phys.-Chim. Sin., 2011, 27: 1108
doi: 10.3866/PKU.WHXB20110504
Korenko M, Vasková Z, Šimko F, et al. Electrical conductivity and viscosity of cryolite electrolytes for solar grade silicon (Si-SoG) electrowinning [J]. Trans. Nonferr. Met. Soc., 2014, 24: 3944
doi: 10.1016/S1003-6326(14)63554-8
9
Suzuki Y, Inoue Y, Yokota M, et al. Effects of oxide ions on the electrodeposition process of silicon in molten fluorides [J]. J. Electrochem. Soc., 2019, 166: D564
doi: 10.1149/2.0441913jes
10
Hu Y J, Wang X, Xiao J S, et al. Electrochemical behavior of silicon (IV) ion in BaF2-CaF2-SiO2 melts at 1573K [J]. J. Electrochem. Soc., 2013, 160: D81
doi: 10.1149/2.038303jes
11
Sakanaka Y, Goto T. Electrodeposition of Si film on Ag substrate in molten LiF-NaF-KF directly dissolving SiO2 [J]. Electrochim. Acta, 2015, 164: 139
doi: 10.1016/j.electacta.2014.12.159
12
De Mattei R C, Elwell D, Feigelson R S. Electrodeposition of silicon at temperatures above its melting point [J]. J. Electrochem. Soc., 1981, 128: 1712
doi: 10.1149/1.2127716
13
Haarberg G M, Famiyeh L, Martinez A M, et al. Electrodeposition of silicon from fluoride melts [J]. Electrochim. Acta, 2013, 100: 226
doi: 10.1016/j.electacta.2012.11.052
14
Monnier R, Giacometti J C. Recherches sur le raffinage électrolytique du silicium [J]. Helv. Chim. Acta, 1964, 47: 345
doi: 10.1002/hlca.19640470202
15
Suzdaltsev A V, Pershin P S, Filatov A A, et al. Review—Synthesis of aluminum master alloys in oxide-fluoride melts: A review [J]. J. Electrochem. Soc., 2020, 167: 102503
doi: 10.1149/1945-7111/ab9879
16
Liu A M, Shi Z N, Xu J L, et al. Preparation of Al-Si master alloy by electrochemical reduction of volcanic rock in cryolite molten salt [J]. JOM, 2016, 68: 1518
doi: 10.1007/s11837-016-1898-x
17
Oishi T, Watanabe M, Koyama K, et al. Process for solar grade silicon production by molten salt electrolysis using aluminum-silicon liquid alloy [J]. J. Electrochem. Soc., 2011, 158: E93
doi: 10.1149/1.3605720
18
Yu X G, Qiu Z X. Preparation of Al-Si alloy by molten salt electrolysis [J]. J. Northeast. Univ. (Nat. Sci.), 2004, 25: 442
Li M, Li Y, Wang Z W. Electrochemical reduction of zirconium oxide and Co-deposition of Al-Zr alloy from cryolite molten salt [J]. J. Electrochem. Soc., 2019, 166: D65
doi: 10.1149/2.1291902jes
20
Bao Morigengaowa. Preparation of Al-Zr alloys by molten salt electrolysis [D]. Shenyang: Northeastern University, 2015
包莫日根高娃. 熔盐电解法制备铝锆合金的研究 [D]. 沈阳: 东北大学, 2015
21
Wang C Z. Solid Electrolyte and Chemical Sensors [M]. Beijing: Metallurgical Industry Press, 2000: 259
王常珍. 固体电解质和化学传感器 [M]. 北京: 冶金工业出版社, 2000: 259
22
Gao Y M, Song J X, Zhang Y Q, et al. Measurement of the diffusivity of oxygen in high-temperature liquid silver by electrochemical method [J]. Acta Metall. Sin., 2010, 46: 277
doi: 10.3724/SP.J.1037.2010.00277
Hu H B, Gao Y M, Lao Y G, et al. Yttria-stabilized zirconia aided electrochemical investigation on ferric ions in mixed molten calcium and sodium chlorides [J]. Metall. Mater. Trans., 2018, 49B: 2794
25
Gao Y M, Yang C H, Zhang C L, et al. Magnesia-stabilised zirconia solid electrolyte assisted electrochemical investigation of iron ions in a SiO2-CaO-MgO-Al2O3 molten slag at 1723 K [J]. Phys. Chem. Chem. Phys., 2017, 19: 15876
doi: 10.1039/C7CP01945A
26
Gao Y M, Huang Z B, He L, et al. Yttria-stabilized zirconia assisted green electrochemical preparation of silicon from solid silica in calcium chloride melt [J]. Metall. Mater. Trans., 2021, 52B: 1708
27
Zaykov Y P, Isakov A V, Zakiryanova I D, et al. Interaction between SiO2 and a KF-KCl-K2SiF6 melt [J]. J. Phys. Chem., 2014, 118B: 1584
28
Monnier R, Barakat D. Contribution à l'étude du comportement de la silice dans les bains de cryolithe fondue [J]. Helv. Chim. Acta, 1957, 40: 2041
doi: 10.1002/hlca.19570400706
29
Bøe G, Grjotheim K, Matiašovský K, et al. Electrolytic deposition of silicon and of silicon alloys Part II: Decomposition voltages of components and current efficiency in the electrolysis of the Na3AlF6-Al2O3-SiO2 mixtures [J]. Can. Metall. Quart., 1971, 10: 179
doi: 10.1179/cmq.1971.10.3.179
30
Bøe G, Grjotheim K, Matiašovský K, et al. Electrolytic deposition of silicon and of silicon alloys Part III: Deposition of silicon and aluminum using a copper cathode [J]. Can. Metall. Quart., 1971, 10: 281
doi: 10.1179/cmq.1971.10.4.281
31
Grjotheim K, Matiašovský K, Fellner P, et al. Electrolytic deposition of silicon and of silicon alloys Part I: Physicochemical properties of the Na3AlF6-Al2O3-SiO2 mixtures [J]. Can. Metall. Quart., 1971, 10: 79
doi: 10.1179/cmq.1971.10.2.79
32
Bøe G, Grjotheim K, Matiašovský K, et al. Electrolytic deposition of silicon and of silicon alloys. Part IV: Preparation of alloys with a high content of silicon, and silicon refining [J]. Can. Metall. Quart., 1972, 11: 463
doi: 10.1179/cmq.1972.11.3.463
33
FactSage. com [EB/OL]. [2021-03-20].
34
Liu A M, Guan J Z, Xie K Y, et al. Aluminothermic reduction-molten salt electrolysis of silica in cryolite melts [J]. J. Northeast. Univ. (Nat. Sci.), 2016, 37: 522
Well D F, Fyfe W S. The 1010 and 800oC isothermal section in the system Na3AlF6-Al2O3-SiO2 [J]. J. Electrochem. Soc., 1964, 111: 582
doi: 10.1149/1.2426187
36
Bao M G, Wang Z W, Gao B L, et al. Solubility of ZrO2 in cryolite-based molten salt system [J]. Adv. Mater. Res., 2014, 968: 67
37
Hou H J, Yang H C, Huangfu G L, et al. Factors affecting the loss on ignition of cryolite [J]. Light Met., 2004, (2): 10
Franke P, Neuschütz D. [Landolt-Börnstein—Group IV Physical Chemistry] Binary Systems. Part 4: Binary Systems from Mn-Mo to Y-Zr|| Si-Zr [M], 2006, 19B4: 1
39
Murray J L, Mcalister A J. The Al-Si (aluminum-silicon) system [J]. Bull. Alloy Phase Diagrams, 1984, 5: 74
doi: 10.1007/BF02868729
40
Chen G Z, Fray D J. Invention and fundamentals of the FFC Cambridge process [A]. Extractive Metallurgy of Titanium [M]. Amsterdam: Elsevier, 2020: 227
41
Gratz E S, Milshtein J D, Pal U B. Determining yttria-stabilized zirconia (YSZ) stability in molten oxy-fluoride flux for the production of magnesium with the SOM process [J]. J. Am. Ceram. Soc., 2013, 96: 3279
doi: 10.1111/jace.12449
